1. Field of the Invention
The present invention relates to a light-emitting device including a single element or a tandem element and a method for manufacturing the light-emitting device.
2. Description of the Related Art
Commercialization of organic electroluminescence (EL) displays is accelerating. Displays are increasingly required to provide high luminance for outdoor use. It is known that the luminance of an organic EL element increases in proportion to electric current and light emission at high luminance can be achieved.
However, a large current flow accelerates deterioration of organic EL elements. Thus, if high luminance can be achieved with a small amount of current, light-emitting elements can have longer lifetime. In this regard, a tandem element in which a plurality of EL layers is stacked has been proposed as a light-emitting element capable of providing high luminance with a small amount of current (see Patent Document 1, for example).
Note that organic EL elements include single elements in which an EL layer including a light-emitting layer is provided between two electrodes (a cathode and an anode), and tandem elements in which two or more EL layers are stacked between two electrodes and an intermediate layer is provided between the EL layers. When n EL layers are stacked in a tandem element, n-fold luminance can be obtained without an increase in current density.
In this specification and the like, a light-emitting layer refers to a layer containing a light-emitting organic compound. A light-emitting layer may be separated into island shapes corresponding to respective elements or may be common to a plurality of elements. An electrode includes a lower electrode, an upper electrode, an anode, and a cathode. An EL layer includes at least a light-emitting layer, and can be further combined with any of a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge generation layer, and the like as appropriate. An intermediate layer includes an electron-injection buffer layer, an electron-relay layer, and a charge generation layer.
Further, a layer with high conductivity described later refers to a layer which has higher conductivity than a light-emitting layer and lower conductivity than an electrode. The layer with high conductivity includes an electron-injection layer, an electron-transport layer, a hole-injection layer, a hole-transport layer, and an intermediate layer. Examples of a material of the layer with high conductivity include a mixed material of an organic compound and a metal oxide, a conductive high molecule material, and the like. In this manner, the EL layer includes a layer with high conductivity and a layer with low conductivity typified by a light-emitting layer.
Note that the electron-injection buffer layer included in the intermediate layer may have an extremely small thickness. A material of the electron-injection buffer layer may be included in part of the EL layer in contact with the electron-injection buffer layer.
A display which includes a single element or a tandem element in which carrier-injection layers with high conductivity (an electron-injection layer and a hole-injection layer), carrier-transport layers (an electron-transport layer and a hole-transport layer), and an intermediate layer are formed has a problem of a crosstalk phenomenon. The crosstalk phenomenon refers to a phenomenon in which current leaks to an adjacent EL element in an adjacent pixel through a layer with high conductivity of the EL element. The crosstalk phenomenon is serious when the width of a partition between EL elements, which is described later, is reduced for the purpose of increasing definition of a display.
In particular, in a tandem element, a plurality of EL layers are stacked with an intermediate layer provided therebetween, and a mixed layer of an organic compound and a metal oxide, a carrier-injection layer with high conductivity including a conductive high molecule, or the like is often used in order to reduce driving voltage; therefore, the tandem element includes layers with high conductivity and layers with low conductivity because of its structure. Furthermore, in the tandem element, electrical resistance between an anode and a cathode is higher than in a single element; thus, current is easily transmitted to an adjacent pixel through the highly conductive layer.
Note that a tandem element is not only the one having a problem of a crosstalk phenomenon. Even in a single element, when a layer with high conductivity such as a carrier-injection layer or a carrier-transport layer is provided in the EL element, current might leak to an adjacent pixel, so that a crosstalk phenomenon might occur.
At least the following patterns can be considered as causes of the crosstalk phenomenon, depending on an element structure and a region where leakage occurs.
1. Leakage through a carrier-injection layer and/or a carrier-transport layer in a single element.
2. Leakage through an intermediate layer in a tandem element.
3. Leakage through a carrier-injection layer and/or a carrier-transport layer in a tandem element.
The current leakage through a carrier-injection layer and/or a carrier-transport layer is caused owing to leakage on part which is closer to a lower electrode than a light-emitting layer is, regardless of whether the carrier-injection layer and/or the carrier-transport layer are/is a hole-injection layer, a hole-transport layer, an electron-injection layer, and/or an electron-transport layer.
<1. Leakage Through Carrier-injection Layer and/or Carrier-transport Layer in Single Element>
FIG. 8 is a schematic diagram for showing a situation where a crosstalk phenomenon occurs in single elements owing to current leakage through a carrier-injection layer and/or a carrier-transport layer with high conductivity, so that adjacent pixels emit light. FIG. 8 illustrates a cross section of a light-emitting panel in which a red single element which emits red (R) light, a green single element which emits green (G) light, and a blue single element which emits blue (B) light are provided in the form of three stripes and shows the state where only the green single element is driven. Note that a crosstalk phenomenon can occur not only in the stripe arrangement but also in a mosaic arrangement, a delta arrangement, and the like.
The light-emitting panel includes the red single element, the green single element, and the blue single element which are adjacent with each other. The red single element is provided between an upper electrode 81 and a first lower electrode 82. The green single element is provided between the upper electrode 81 and a second lower electrode 83. The blue single element is provided between the upper electrode 81 and a third lower electrode 84.
A hole-injection-transport layer 90, a light-emitting layer 91, and an electron-transport-injection layer 92 are stacked in this order in each of the red, green, and blue single elements.
In the case where the upper electrode 81 has a light-transmitting property, a counter glass substrate 88 can be provided over the upper electrode 81, and a reflective electrode can be used as the lower electrode. The counter glass substrate 88 may include a red color filter, a green color filter, and a blue color filter although not illustrated. The red color filter, the green color filter, and the blue color filter overlap with the first lower electrode 82, the second lower electrode 83, and the third lower electrode 84, respectively.
When only the green single element is driven in the above-described light-emitting panel by application of a voltage between the second lower electrode 83 and the upper electrode 81, current might leak to the adjacent red or blue single element through the hole-injection-transport layer 90 with high conductivity, causing red line (the red single element) or blue line (the blue single element) to emit light and a crosstalk phenomenon to occur. Note that electrons flow as shown by arrows 93 and holes flow as shown by arrows 94.
<2. Leakage Through Intermediate Layer in Tandem Element>
FIG. 9 is a schematic diagram for showing a situation where a crosstalk phenomenon occurs in tandem elements owing to current leakage through an intermediate layer 86 having high conductivity, so that adjacent pixels emit light. FIG. 9 illustrates a cross section of a light-emitting panel (white panel) including tandem elements arranged in the form of three stripes and configured to emit white light, showing the state where only a second tandem element is driven.
The light-emitting panel includes first to third tandem elements which are adjacent to each other. The first tandem element is provided between the upper electrode 81 and the first lower electrode 82. The second tandem element is provided between the upper electrode 81 and the second lower electrode 83. The third tandem element is provided between the upper electrode 81 and the third lower electrode 84.
In each of the first to third tandem elements, a first EL layer 85, the intermediate layer 86, and a second EL layer 87 are stacked in this order. For example, when the first EL layer 85 includes a light-emitting layer capable of emitting blue light and the second EL layer 87 includes a light-emitting layer capable of emitting green light and a light-emitting layer capable of emitting red light, each tandem element can provide white light emission.
In the case where the upper electrode 81 has a light-transmitting property, the counter glass substrate 88 can be provided over the upper electrode 81, and a reflective electrode can be used as the lower electrode. The counter glass substrate 88 is provided with a blue color filter, a red color filter, and a green color filter which are not illustrated. The red color filter, the blue color filter, and the green color filter overlap with the first lower electrode 82, the second lower electrode 83, and the third lower electrode 84, respectively.
When only the blue line (the second tandem element) is driven in the above-described light-emitting panel by application of a voltage between the second lower electrode 83 and the upper electrode 81, current might leak to the adjacent first or third tandem element through the intermediate layer 86 with high conductivity, causing the red line (the first tandem element) or green line (the third tandem element) to emit light and a crosstalk phenomenon to occur.
<3. Leakage Through Carrier-injection Layer and/or Carrier-transport Layer in Tandem Element>
FIG. 10 is a schematic view for showing a situation where a crosstalk phenomenon occurs in tandem elements owing to current leakage through a carrier-injection layer and/or a carrier-transport layer (a hole-injection and/or a hole-transport layer) 89 with high conductivity, so that adjacent pixels emit light, illustrating a state where only the blue line (the second tandem element) is driven in the light-emitting panel (a white panel).
In each of first to third tandem elements, a first EL layer 85 including the carrier-injection layer and/or the carrier-transport layer 89, the intermediate layer 86, and the second EL layer 87 are stacked in this order.
Note that in the tandem elements, current leakage to adjacent pixels through each of the intermediate layer 86 and the carrier injection layer and/or carrier-transport layer 89 with high conductivity, and another layer with high conductivity might occur concurrently.
A conventional technique for preventing generation of a crosstalk phenomenon is described below.
<Conventional Technique 1 (See Patent Document 2>
FIG. 11 is a cross-sectional view schematically illustrating a light-emitting device of a conventional technique 1. First to third lower electrodes 82 to 84 are formed over a substrate 70, and partitions 72 are provided between the first to third lower electrodes 82 to 84. An EL layer 71 including a layer with high conductivity and a light-emitting layer is formed over the partitions 72 and the first to third lower electrodes 82 to 84 by an evaporation method. An upper electrode 81 is formed over the EL layer 71.
According to the conventional technique 1, the thickness of the EL layer 71 formed over a slope 74 by an evaporation method is reduced by increasing an inclination angle of the slope 74 of the partition 72, whereby the thickness of the layer with high conductivity included in the EL layer 71 is reduced. That is, the thickness of the layer with high conductivity included in the EL layer 71 formed over the slope 74 is set to smaller than the thickness of the layer with high conductivity included in the EL layer 71 formed over the first to third lower electrodes 82 to 84. Thus, the resistance of the layer with high conductivity included in the EL layer 71 formed over the slope 74 of the partition 72 can be larger than the resistance of the layer with high conductivity included in the EL layer 71 formed over the first to third lower electrodes 82 to 84. As a result, current can be prevented from leaking to an adjacent light-emitting element through the layer with high conductivity formed over the partition 72, so that generation of a crosstalk phenomenon can be prevented.
Note that when the inclination angle of the slope 74 of the partition 72 is increased, the upper electrode 81 formed over the slope 74 is also thinned, so that disconnection might occur or the resistance of the film might be increased; as a result, defective lighting might occur.
<Conventional Technique 2 (see Patent Document 3)>
FIG. 12 is a cross-sectional view schematically illustrating a light-emitting device of a conventional technique 2. First to third lower electrodes 82 to 84 are formed over a substrate 70, and partitions 72 are provided between the first to third lower electrodes 82 to 84. An EL layer 71 in which a layer with high conductivity and a light-emitting layer are stacked is formed over the first to third lower electrodes 82 to 84. The EL layer 71 is not formed over the partitions 72. An upper electrode 81 is formed over the partitions 72 and the EL layer 71.
According to the conventional technique 2, since the EL layer 71 is not formed over the partitions 72, a situation where current leaks to an adjacent light-emitting element through a layer with high conductivity which is formed over the partition 72 does not arise; as a result, generation of a crosstalk phenomenon can be prevented.
Note that in order not to form the EL layer 71 over the partitions 72, the EL layer 71 needs to be formed selectively over the first to third lower electrodes 82 to 84 by an evaporation method, an ink-jet method, or the like using a mask. Therefore, an evaporation method has a disadvantage that manufacturing cost is high at the time of manufacturing a high definition panel because an expensive high definition mask is needed. Further, an ink-jet method has a disadvantage that control of an impact position is difficult, whereby an yield is reduced.